Phosphaphenanthrene‐containing borate ester as a latent hardener and flame retardant for epoxy resin

Borate ester containing the phosphaphenanthrene group with N → B coordination structure (PBN) was synthesized by transesterification of tributyl borate, 2‐(6‐oxido‐6H‐dibenz⟨c,e⟩⟨1,2⟩oxaphos‐phorin‐6‐yl) methanol and N,N‐dimethylethanolamine. A thermally latent curing utility for diglycidyl ether of bisphenol A epoxy resin (E51) was confirmed by differential scanning calorimetry. Additionally, its flame‐retarding function in the cured epoxy was demonstrated in terms of the limiting oxygen index (LOI) and vertical burning test. The cured epoxy with 100:20 mass ratio of E51 to PBN passed UL94 V‐0 rating with 34.3% of LOI. The flame retardation mode and thermal and mechanical properties of the cured epoxy were carefully evaluated. The results of this work suggest that application of PBN would permit the formulation of environmentally friendly one‐pot flame‐retardant epoxy resin.